The compressors of gas turbine engines are generally provided with variable stator vanes, especially compressors which have relatively high pressure ratios, to ensure that the compressor will operate efficiently over its full speed range. The variable stator vanes are used to correct the angle of incidence of the air onto a stage of rotor blades to angles which they can tolerate without a break down of flow, stall or surge at relatively low compressor pressure speeds.
A variable stator vanes angular position is controlled by an operating lever, which is connected to a control ring positioned generally coaxially with the compressor casing. The control ring is usually moved, or rotated, by a ram so as to adjust the positions of the variable stator vanes.
The control ring rotates on low friction support pads, which are mounted on the compressor casing to control the position and shape of the control ring under load.
However, if the control ring becomes distorted, or becomes eccentric, then the operating levers will move by differing amounts and the variable stator vanes will take up different angular positions. The different angular positions of the variable stator vanes affect the performance of the compressor and hence the performance of the gas turbine engine.
It is desirable to have as small a radial clearance as possible between the compressor casing, or more accurately the low friction pads, and the control ring in order to minimise error or discrepancy of the variable stator vane angular position. In operation the compressor casing temperature is higher than the control ring and therefore in operation the compressor casing expands more than the control ring, decreasing the clearance between the compressor casing and the control ring. The radial clearance is initially set to allow for tolerances and for the differential thermal growth between the compressor casing and the control ring in order to prevent binding between the control ring and the low friction pads.
These requirements result in an increased radial clearance between the compressor casing and the control ring, which increases the error or discrepancy in the variable stator vane angular position.